DRILLING PREDATION IN THE CALOOSAHATCHEE FORMATION (PLIO-PLEISTOCENE OF FLORIDA): TEST OF A LATITUDINAL PREDATION GRADIENT

Spatial variation in predation by drilling gastropods remains inadequately understood, with some previous work indicating an increase, and other studies a decrease, in drilling with latitude. Work by Kelley and Hansen on Recent drilling predation revealed greater drilling in the Carolinian Province compared to the Gulf Province. To investigate latitudinal changes of predation in the fossil record, drilling frequency (DF) and prey effectiveness (PE) were calculated for a bulk sample of the Caloosahatchee Formation (Plio-Pleistocene of La Belle, Florida). Drilling was examined for the bivalve fauna as a whole and for several families, genera and species shared with assemblages from the middle Atlantic Coastal Plain. Because the Caloosahatchee has been correlated to both the Chowan River (late Pliocene) and the James City/Waccamaw (early Pleistocene) Formations, comparisons of DF and PE were made between each of these formations and the Caloosahatchee.

Of 7126 Caloosahatchee specimens examined, only 129 were drilled, 11 of which were edge-drilled, yielding a drilling frequency of 3.6% (with edge-drilling removed, 3.3%). This value is significantly less than those for both the James City/Waccamaw (13%) and the Chowan River (10%). Within the Caloosahatchee, DF for the Crassatellidae and Corbulidae and the genus Caryocorbula is 0%; DF for Arcidae, Lucinidae, and Carditidae is 5.7%, 3.6%, and 2.0% respectively. In all cases, DF for the Caloosahatchee is lower than that of the Chowan River and the James City/Waccamaw Formations, but the difference is statistically significant only for the lucinids (16.6% in the Chowan River and 28.1% in the James City/Waccamaw). In addition, drilling on the common species Parvilucina multilineata was significantly greater in the middle Atlantic Coastal Plain than in Florida (~20% vs. 3%). As with DF, PE of the Caloosahatchee is low: 0.03 (edge drilling excluded), compared to 0.13 and 0.16 for the Chowan River and James City/Waccamaw.

Regardless of which formation correlates with the Caloosahatchee, DF decreases at lower latitudes, consistent with our observations in the Recent and suggesting that drilling is less common at lower latitudes where drillers are at greater risk from enemies. However, the lower value of PE in the Caloosahatchee is not consistent with this explanation.